1. Field of the Invention
The invention relates to a method of manufacturing monodisperse poly(meth)acrylate particles of stable shape, having diameter 1-20 micron.
2. Discussion of Related Art
The evidence in the industry indicates a growing demand for plastics in the form of elastic particles of stable shape having a defined, uniform particle size in the range 2-20 micron. Such particles are used, e.g., as spacers (e.g. in displays and films), surface modifying agents, support materials in diagnostics, etc.
However, the primary interest is in the area of the optics industry, in which particles in this size range having an index of refraction which can be precisely adjusted with respect to the index of refraction of a given polymer matrix can be used to achieve various optical effects.
In this connection, the profile of required properties for such particles with diameter 5-15 micron has long been known; however, there has not been available a practicable method of manufacturing such particles. The classical method of manufacturing defined particles, emulsion polymerization, does not succeed in this particle range (see 1992, "Ullmanns Encyclopedia of Industrial Chemistry"., 5th Ed., Vol. A21, pub. VCH, pp. 168, 373-387; Becker and Braun, 1990, "Kunststoff-Handbuch", Vol. 1, pub. Carl Hanser, pp. 26-28). In general, emulsion polymerization can be used to produce particles with diameter .ltoreq.2 micron, hut attempts to produce larger particles are accompanied by problems, in particular formation of new particles, leading to multimodal particle size distributions. According to literature data it should be possible to manufacture particles of the stated size by repetitive absorption of aqueous dispersions containing monomers (see Ugelstad, J., Mork, P. C., Kaggurud. K. H., Ellingsen, T. and Berge, A., 1980 Adv. Colloid Interface Sci. 13, 191).
However, the method described is very complex. Another method, wherein the subject particles are manufactured in an environment with microgravity (in a space shuttle in space), holds little promise for industrial exploitation (see Venderhoff, J. W., El-Asser., M. S., Micale, F. J.1, Sudol., E. D., Tsena, C. M., Silwanowicz, A., Sheu, H. R., and Kornfeld. D. M., 1986 P - Mater. Sci. Eng. Prepr. 54, 587). Thus it is concluded that heretofore no simple, industrially applicable method existed for manufacturing such particles in water as the reaction medium. Also, classical suspension polymerization technique, wherein it is well known that particle size is controlled primarily by the stirring speed, generally does not yield particles in the size range 5-15 micron. Moreover, these particles are not monodisperse, but have a wide particle size distribution.
The principal applications of these particles are light scattering applications wherein the index of refraction of the particles is an important factor (see Jap. Pat. App. 03-126,766; Chem. Abstr. 115, 209446n). Particles having a core-and-shell structure in this size range are frequently used for, e.g., matt coatings (Jap. Pat. App. 03-58,840; Chem. Abstr. 115, 116478; Eur. OS 342,283).
The possibility is more favorable of obtaining monodisperse PMMA particles with diameter 2-20 micron by producing the particles by the principle of precipitation polymerization in an organic medium, with the use of an organic dispersant.
There have been a relatively large number of publications on this subject. Precipitation polymerization of PMMA in hydrocarbons as solvents was proposed as early as the 19308 (U.S. Pat. No. 2,135,443, Ger. Pat. 662,157). Since then over 100 patent-type publications and numerous other publications have appeared which deal with polymerization of alkyl (meth)acrylates in non-aqueous dispersions.
In many of the patent-type publications mentioned, the applications described relate purely to paints and similar coatings, involving stable organic dispersions of very fine particles. There are also publications reporting investigation of the effect of emulsifiers, initiators, and solvent grade on the particle size. A very informative summary of dispersion polymerization of methyl methacrylate in non-aqueous media is provided in Winnik, M. A. et al., 1987, Makromol. Chem. Macromol. Symp. 10/11, 483-501.
Block copolymers are the most prominent emulsifiers used for dispersion polymerization. An overview of currently used polymerization conditions is provided in Winnik, M. A., et al., loc. cit., p. 485 (Table 1).
It may also be seen from Winnik, M. A., et al., loc. cit., that the particle size is controllable via the emulsifier concentration (FIG. 1), the initiator concentration (FIG. 5), and the solids content (FIG. 3) and solvent grade (FIG. 4). The graphics presented therein indicate that it is particularly possible to control in favor of larger particles (&gt;3 micron) with the use of mixtures of tetrachloromethane and alkanes. If halogenated hydrocarbons are not employed, regimes are encountered in which it is not possible to control particle size; instead, coagulation occurs.
The use of halogenated hydrocarbons in industry can no longer be justified, because of deleterious ecological and toxicological effects. Accordingly, there is a need as described above, for means of producing monodisperse poly(meth)acrylate particles, preferably in the range of particle sizes of 1-20 micron, without the use of objectionable substances such as halogenated hydrocarbons. This problem is solved by the inventive method, which prescribes a specific formulation.